The present invention relates to high-Cr steel pipes for line pipes which are used for transport of oil or natural gas, and which exhibit superior toughness at low temperatures.
In recent years, oil and natural gas wells which could be easily drilled have been exhaustively drilled, and wells in severe drilling environments, such as severely corrosive environments, deep wells, cold environments, and sea bottoms, have unwillingly start to be drilled.
Oil and natural gas produced from such wells contain large amounts of carbon dioxide gas in many cases, and low-carbon steel or low-alloy steel is significantly corroded in such environments. Thus, addition of inhibitors has been performed as an anticorrosion measure.
The use of inhibitors, however, increases cost and results in insufficient effects at high temperatures, and leakage thereof causes environmental contamination. Thus, corrosion resistant alloys (CRA) not requiring inhibitors have tended to be used recently. In oil tubular country goods (OCTG), martensitic stainless steel containing 13% of chromium (Cr) is widely used as such an CRA.
On the other hand, in line pipes, low-carbon martensitic stainless steel containing 12% Cr is defined in the API standard. This steel requires preheating and post welding heat treatment (PWHT) for girth welding, resulting in increased cost, and exhibits poor toughness at the welds. Thus, this steel has not substantially been employed as line pipes.
Accordingly, as a CRA for line pipe material, chromium-enriched duplex stainless steel containing nickel and molybdenum has been used due to the good weldability and corrosion resistance thereof.
A problem with duplex stainless steel, however, is excessive quality for some wells, resulting in increased cost.
In order to solve this problem, Japanese Unexamined Patent Application Publication No. 8-295939 discloses a method for making a high-Cr martensitic steel pipe for line pipes including forming a pipe from 10 to 14%-Cr steel and heat-treating this under specific conditions in which carbon (C) and nitrogen (N) are reduced to 0.03% or less and 0.02% or less, respectively, and copper (Cu) is adjusted to 0.2 to 1.0%. This method provides a steel pipe which is superior in corrosion resistance in a carbon dioxide gas environment, in weldability, and in heat-affected-zone (HAZ) toughness.
In this method, however, toughness is improved by heat treatment. Thus, the toughness is limited in the HAZ in which the heat treatment effects are lost, and the steel pipe does not satisfy requirements for higher toughness. Moreover, in the disclosed composition, the steel exhibits insufficient hot workability, and numerous defects occur in the production of seamless pipes.
An object of the present invention is to provide a high-Cr steel pipe for line pipes which exhibits further improved HAZ toughness and hot workability by the application of optical chemical composition, in view of the problems in the conventional art.
The present inventors have intensively studied in order to achieve the above object and have discovered that, as shown in FIG. 1, toughness at low temperatures is improved by decreasing carbon to 0.02% or less and increasing nickel from conventional 1.5% to more than 2.0%, that addition of Nb thereto improves toughness, and that this composition exhibits Heat-Affected-Zone (HAZ) toughness and hot workability which are significantly superior to those of conventional compositions. Herein, FIG. 1 shows the relationship between the yield strength (YS) and the fracture appearance transition temperature (50% FATT) obtained by the arrangement of the results of the tensile test and the Charpy impact test of steel pipes, which were made by followings; the raw materials having the compositions shown in the drawing was heated and was rolled to a seamless pipe of a diameter of 273 mm and a thickness of 13 mm, and the pipe was cooled in air to room temperature, was heated to the Ac3 point or more for quenching, and was tempered at less than the Ac3 point.
Pipelines are generally subjected to cathodic protection to prevent corrosion on the outer surface of steel pipes. Cathodic polarization of the steel pipes by sacrificial anodes such as Zn alloys or external power supply prevents the anodic reaction of iron. It is, however, feared that overprotection (a state of cathodic polarization at a potential which is more disnoble than a level required for protection) results in embrittlement due to hydrogen generated by the cathodic reaction. Thus, steel pipes require high resistance to hydrogen embrittlement, assuming that overprotection occurs by any cause.
The steel in accordance with the present invention does not occur hydrogen embrittlement cracks and exhibit high resistance to hydrogen embrittlement, as well as low-Ni materials (up to 2.0%) of comparative examples.
FIG. 2 shows the relationship between the amount of the permeated hydrogen and the Ni content when a hydrogen permeation test (the thickness of the test piece: 1.0 mm and the permeation area: 7 cm2) is performed in a simulated environment of overprotection.
An increase in Ni to 2.0 to 3.0% causes a decrease in the amount of the permeated hydrogen. That is, resistance to hydrogen embrittlement is further improved. The steel No. agrees with the steel type shown in Table 1 in Examples.
The present invention, completed by further investigations based on these results, relates to a high-Cr steel pipe for line pipes having a composition comprising, by percent by weight:
C: 0.02% or less,
Si: 0.5% or less,
Mn: 0.2 to 3.0%,
Cr: 10.0 to 14.0%,
Ni: more than 2.0 to 3.0%,
N: 0.02% or less, and
the balance being Fe and incidental impurities.
In the present invention, the composition preferably further comprises 0.3% or less of Nb.
In the present invention, the composition may further comprises at least one of the following (a) to (c):
(a) V: 0.3% or less,
(b) Cu: 1.0% or less, and
(c) at least one of Ti, Zr, and Ta in a total amount of 0.30% or less.